Capacitive touch digitizing tablet systems that allow a user to operate a computing device with a stylus and/or fingertip are known. Typically, a digitizer is integrated with a display screen, e.g. over-laid on the display screen, to correlate user input, e.g. physical touch or stylus interaction on the screen with the virtual information portrayed on it. Position detection of the stylus and/or finger tip provides input to the computing device and is interpreted as user commands. In some embodiments of the present invention, the capacitive touch digitizing system is configured only for touch, e.g. capacitive touch digitizing system for mobile telephones.
U.S. Pat. No. 6,690,156 entitled “Physical Object Location Apparatus and Method and a Platform using the same” assigned to N-trig Ltd, and U.S. Pat. No. 7,292,229 entitled “Transparent Digitizer” also assigned to N-Trig Ltd, both of which are hereby incorporated by reference in their entirety, describe a digitizing tablet system capable of detecting position of a user input, e.g. physical object, game pieces and/or a styluses, including an electrical circuit, either active or passive. Typically, the system includes a transparent digitizer overlaid on a Flat Panel Display (FPD). The digitizer includes a matrix of vertical and horizontal conducting lines and sensors to sense an electric signal passing through the vertical and horizontal conductive lines. Positioning the physical object at a specific location on the digitizer provokes a signal whose position of origin may be detected.
U.S. Pat. No. 6,750,852 entitled “Object position detector with edge motion feature and Gesture Recognition” assigned to Synaptics, Inc. which is hereby incorporated by reference in its entirety, describes a method for recognizing a gesture made on a touch-sensor pad in a touch-sensing system providing X and Y position information to a host.
U.S. Pat. No. 6,750,852 additionally describes an adaptive analog technique to overcome offset and scale differences between channels. According to the disclosure in U.S. Pat. No. 6,750,852, calibration is performed in real time (via long time constant feedback) to zero out any long term effects due to sensor environmental changes. Calibration is based on offset values read when no object is thought to be present at the touch sensor. The absence of an object is determined by comparing sensed pressure measurements to pressure measurements that may be typical of measurements when no finger is present.
U.S. Patent Application Publication No. 20040155871 entitled “Touch Detection for a Digitizer” assigned to N-trig Ltd, which is hereby incorporated by reference in its entirety, describes a digitizing tablet system capable of detecting position of both physical objects and finger touch using the same sensing conductive lines. Typically, the system includes a transparent sensor overlaid on a Flat Panel Display (FPD). The digitizer's sensor includes a matrix of vertical and horizontal conducting lines to sense an electric signal passing through the vertical and horizontal conductive lines. Touching the digitizer in a specific location provokes a signal whose position of origin may be detected.
U.S. Patent Application Publication No. 20040155871 additionally describes parasitic capacitance developed between the display screen and the conductive lines of the overlaying digitizer sensor. Slight differences in distance between the digitizer and screen, material structure in specific areas of the digitizer screen, environmental conditions and parasitic capacitance on associated PCB, may affect the parasitic capacitance level between the screen and some of the lines. The unbalanced capacitance creates an unbalance steady state signal the lines, e.g. an offset signal and/or background signal. The result is a different steady state signal on each of the lines that will result in an amplified non-zero steady state signal being produced by the differential amplifier. In some exemplary embodiments, detection is based on an AC signal crossing at junctions of conductive lines by virtue of the capacitance of the finger. In such embodiments, parasitic capacitance may result in a different steady state signal on each of the junctions. The presence of these offset signals may reduce the level of accuracy possible in detecting, for example, a user's finger's location.
U.S. Patent Application Publication No. 20040155871 describes a mapping solution that may be used to compensate for the display panel offset signal phenomenon. During an initialization procedure, magnitude and phase of the difference signals for each pair of conducting lines connected to a differential amplifier is detected and stored. Once the differential map is stored in memory, it can be used to compensate for the display panel offset signal phenomenon. Offset in the signal may cause saturation when sampling the signal that cannot be compensated for with the mapping solution.
U.S. Patent Application Publication No. 20060227114 entitled “Touch location determined with error correction for sensor movement” assigned to 3M Innovative Properties Company, which is hereby incorporated by reference in its entirety, describes a touch sensitive device including a capacitive touch sensor configured to generate signals indicative of a location of a capacitively coupled touch on a touch surface, an error correction sensor configured to generate a signal associated with an error in the touch location signals, the error associated with movement of the capacitive touch sensor; and a processor configured to determine the touch location based on the touch location signals and the error signal.
U.S. Patent Application Publication No. 20060267953 entitled “Detection of and compensation for stray capacitance in capacitive touch sensors” assigned to 3M Innovative Properties Company, which is hereby incorporated by reference in its entirety, describes a system and method for detecting stray capacitance in a touch sensor system. The system includes a capacitive touch sensor having a resistive layer covering an active area, and electronics coupled to a plurality of locations on the resistive layer for determining touch position based on currents flowing through the plurality of locations due to capacitive coupling of a touch implement with the resistive layer. The method includes determining a test touch position when a detected signal level exceeds a threshold value and using the test touch position to determine existence or absence of stray capacitance.
U.S. Patent Application Publication No. 20070268272 entitled “Variable Capacitor Array” assigned to N-trig Ltd, which is hereby incorporated by reference in its entirety, describes a digitizer sensor including pairs of conductive lines coupled to differential amplifiers through which a difference signal is detected, and capacitors operative to balance differences in parasitic capacitance between the conductive lines.